Cryogenic refrigerator

ABSTRACT

A cryogenic refrigerator for cooling a rotating device includes a stationary regenerator, and a rotatable cold heat exchanger coupled to the stationary regenerator to rotate relative thereto. The cryogenic refrigerator is, for example, of the Gifford-McMahon type or pulse tube type. In the Gifford-McMahon type, a stationary cylinder houses the regenerator, and a rotatable cylinder mounted to the cold heat exchanger is concentrically arranged about the stationary cylinder. Alternatively, the rotatable cylinder is axially offset of the stationary cylinder. A seal, for example, a ferrofluidic seal, is located between the stationary and rotatable cylinders. In the pulse-tube type, a pulse tube is concentrically arranged about the regenerator, and the cold heat exchanger includes a stationary portion coupled to the regenerator and a rotatable portion coupled to the pulse tube. A back-up valve system is provided for increased reliability.

BACKGROUND

This invention relates to cryogenic refrigerators.

Gifford-McMahon and pulse-tube cryocoolers are known sources ofcryogenic refrigeration for cooling superconductor devices. Where thesuperconductor device is rotating, such as in a superconductor motor, athermal link, for example, a fan, is provided to couple the stationarycryogenic refrigerator to the rotating device.

SUMMARY

According to one aspect of the invention, a cryogenic refrigerator forcooling a rotating device includes a stationary regenerator, and arotatable cold heat exchanger coupled to the stationary regenerator torotate relative thereto.

Embodiments of this aspect of the invention may include one or more ofthe following features.

The cryogenic refrigerator is of the Gifford-McMahon type. A stationarycylinder houses the regenerator, and a rotatable cylinder mounted to thecold heat exchanger is concentrically arranged about the stationarycylinder. A filler material is located between the stationary androtatable cylinders.

In an illustrated embodiment, the rotatable cylinder is axially offsetof the stationary cylinder and aligned along a common axis. A stemextends from the regenerator. The cylinders define a flow channeltherebetween.

A seal, for example, a ferrofluidic seal, is located between thestationary and rotatable cylinders.

In another illustrated embodiment, the cryogenic refrigerator is of thepulse-tube type with a pulse tube concentrically arranged relative tothe regenerator, for example, the pulse tube is concentrically arrangedabout the regenerator. The cold heat exchanger includes a stationaryportion coupled to the regenerator and a rotatable portion coupled tothe pulse tube. The stationary and rotatable portions of the cold heatexchanger define a flow channel therebetween, and the stationary portiondefines a flow channel. The cold heat exchanger includes screens. Thecryogenic refrigerator includes a surge volume housing, an aftercooler,and a warm end heat exchanger. The surge volume housing and theaftercooler define a flow orifice therebetween.

According to another aspect of the invention, a method of cooling arotating superconductor device includes providing a cryogenicrefrigerator including a stationary regenerator and a rotatable coldheat exchanger coupled to the stationary regenerator to rotate relativethereto, and coupling the rotatable cold heat exchanger to thesuperconductor device.

According to another aspect of the invention, a pulse tube cryogenicrefrigerator includes first and second valve assemblies for controllingflow between a compressor and a regenerator of the refrigerator, and acontroller for detecting failure in the first valve assembly andswitching from the first valve assembly to the second valve assembly.

Embodiments of this aspect of the invention may include one or more ofthe following features.

Each valve assembly includes a rotary valve including a high pressureflow channel and a low pressure flow channel. Alternatively, each valveassembly includes first and second solenoid valves. The pulse tubecryogenic refrigerator includes a valve, for example, first and secondsolenoid valves, for switching between the first and second valveassemblies, and first and second differential transducers for measuringpressure across the valve assemblies.

Advantages of the invention include the ability to directly couple therefrigerator to a rotating object to cool the rotating object withouthaving to rotate the refrigerator regenerator. Additional advantagesinclude a back-up valve system providing reliability in case of systemfailure.

Other features, objects, and advantages of the invention will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional side view of a Gifford-McMahon typecryogenic refrigerator;

FIG. 2 is a cross-sectional side view of an additional embodiment of aGifford-McMahon type cryogenic refrigerator;

FIG. 3 is a cross-sectional side view of a pulse tube cryogenicrefrigerator; and

FIG. 4 is a schematic of a pulse tube cryogenic refrigerator including asecondary valve assembly.

DETAILED DESCRIPTION

Referring to FIG. 1, a cryogenic refrigerator 10, generally of theGifford-McMahon type, includes a compressor 12 and a cold head 14connected by inlet and exhaust lines 16, 18, controlled respectively byinlet and exhaust valves 20, 22, for example, single rotary valves. Coldhead 14 has a warm end 14 a and a cold end 14 b, and includes an inner,stationary cylinder 24, a displacer/regenerator assembly 26 axiallymovable within cylinder 24 (in the direction of arrow, A), an outer,rotatable cylinder 28, and a cold heat exchanger 30 mounted to rotatewith outer cylinder 28 (arrow, B). Cylinder 28 is concentricallyarranged about cylinder 24 and is rotatable relative to cylinder 24.

Cylinder 24 defines an upper end volume 34 with gas being delivered toand received from upper end 34 of cylinder 24 through channels 31defined by a control disk 41 mounted to a control stem 32 ofdisplacer/regenerator assembly 26. Channels 31 communicate with inletand exhaust lines 16, 18 via lines 19. Displacer/regenerator assembly 26includes an axially extending stem 60 for gas flow between assembly 26and cold heat exchanger 30. Cylinder 24 has at its lower end 36 openings38 which permit cooled gas to pass from heat exchanger 30 into anexpansion space 62.

Mounted to cylinder 24 at warm end 14 a is a housing 40 that enclosesvalves 20, 22. Cylinder 24 and housing 40 include flanges 42, 44,respectively, with a seal 46, for example, an O-ring seal, positionedtherebetween. Between displacer/regenerator 26 and cylinder 24 arefurther seals 48 and 50, for example, O-ring seals, and between controlstem 32 and control disk 41 is a further seal 52, for example, an O-ringseal. At warm end 14 a of cold head 14, between the stationary androtating cylinders 24, 28 is a warm ferrofluidic seal 54 and O-ring 54a. Between the two cylinders 24, 28 is a space 56 filled with a fillermaterial, for example, foam, to reduce heat losses from warm end 14 a tocold end 14 b. Space 56 has a thickness, for example, of a couple mils.

In use, rotatable cylinder 28 is coupled at cold end 14 b to a rotatingmachine (not shown) to rotate therewith. Coolant is delivered to heatexchanger 30 by cycling gas within cold head 14, as follows. Withdisplacer/regenerator 26 positioned at lower end 36 of cylinder 24,inlet valve 20 is opened and the pressure in upper end volume 34 abovedisplacer/regenerator 26 is increased from a first pressure P₁ to asecond, higher pressure P₂. The volume below displacer/regenerator 26 ispractically zero during this process because displacer/regenerator 26 isat its lowest position. With inlet valve 20 still open and exhaust valve22 still closed, the displacer/regenerator 26 is moved to the top ofcylinder 24. This action moves the gas that was originally in volume 34down through the displacer/regenerator 26 to expansion space 62. The gasis cooled as it passes through displacer/regenerator 26, decreasing involume and thus causing more gas to be drawn into cylinder 24 throughinlet valve 20 to maintain a constant pressure within the system.

With displacer/regenerator 26 at the top of cylinder 24, inlet valve 20is closed and exhaust valve 22 is opened, allowing the gas within lowerexpansion space 62 to expand to the initial pressure P₁ as gas escapesfrom cylinder 24 through exhaust valve 22. Gas that remains within lowerspace 62 has done work to push out the gas that escapes during thisprocess. Energy is thus removed from the gas that remains in lower space62, causing the gas remaining in lower space 62 to drop to a lowertemperature. The low temperature gas is forced from lower space 62through heat exchanger 30 by moving displacer/regenerator 26 downward tothe bottom of cylinder 24. Heat is transferred to the gas in heatexchanger 30 from the low temperature source, e.g., a superconductormagnet or high-temperature superconductor coil. The gas flows from heatexchanger 30 back through displacer/regenerator 26, in which the gas iswarmed back to near ambient temperature.

Other embodiments are within the scope of the following claims.

For example, referring to FIG. 2, a cryogenic refrigerator 110,generally of the Gifford-McMahon type, includes a compressor 12 and acold head 114 connected by inlet and exhaust lines 16, 18, controlledrespectively by inlet and exhaust valves 20, 22. Cold head 114 includesan upper, stationary cylinder 124, a displacer/regenerator assembly 126axially movable within cylinder 124, a rotatable cylinder 128 arrangedaxially below cylinder 124 along a common axis, Z, and a cold heatexchanger 130 mounted to rotate with lower cylinder 128.Displacer/regenerator assembly 126 includes an axially extending stem160 for gas flow between assembly 126 and cold heat exchanger 130. Alower section 124 b of cylinder 124 defines openings 138 which permitcooled gas to pass from heat exchanger 130 into an expansion space 162.

At a lower end 124 a of stationary cylinder 124, between stationary androtating cylinders 124, 128, is a ferrofluidic seal 154 and O-ring 154a. Cylinders 124, 128 include extensions, 162, 164, respectively, whichdefine a long, thin flow channel 166, at the end of which is locatedseal 154 to distance seal 154 from the coolant to limit heating of thecoolant by seal 154. A filler 170, for example, a teflon tube to limitfluid leak, is located between lower, stationary cylinder section 124 band an inner, rotating section 128 a of lower cylinder 128.

Referring to FIG. 3, a pulse tube refrigerator 210 includes a rotatablecold end heat exchanger 224 for direct coupling to a cryogenic rotatingdevice, not shown. Pulse tube refrigerator 210 includes the followingstationary components: a pressure wave generator 212, a valve system 214connecting to pressure wave generator 212, an aftercooler 216, aregenerator 218, and a warm end heat exchanger 220. Mounted to rotaterelative to regenerator 218 is a pulse tube 222. Cold end heat exchanger224 has a stationary portion 224 a mounted to regenerator 218 and arotatable portion 224 b mounted to pulse tube 222 to rotate therewith.Mounted to pulse tube 222 at the warm end 222 a of the pulse tube torotate therewith is a housing 226 enclosing a surge volume 228. Pulsetube 222 and regenerator 218 form a co-axial pulse tube, as described,for example, in Richardson, R. N., “Development of a Practical PulseTube Refrigerator: Co-axial Design and influence of Viscosity,”Cryogenics, Vol. 28, No. 8, p. 516, incorporated by reference herein.

Stationary portion 224 a of cold end heat exchanger 224 defines a flowchannel 230 in fluid communication with a channel 232 defined betweenstationary and rotating portions 224 a, 224 b of cold end heat exchanger224. Channel 232 is in fluid communication with pulse tube 222. Cold endheat exchanger 224 includes a screen 234 located between a bottom end236 of regenerator 218 and stationary portion 224 a of cold end heatexchanger 224. The narrow flow channels and screen form a large surfacearea providing high convective heat transfer.

Between the rotatable surge housing 226 and the stationary aftercooler216 at warm end 222 a of pulse tube 222 is a clearance 240, which actsas a fluid orifice allowing the gas from pulse tube 222 to travel tosurge volume 228. The size of clearance 240 is selected to properly tunepulse tube refrigerator 210, as discussed, for example, in Ohtani etal., U.S. Pat. No. 5,412,952, incorporated by reference herein. For atypical application in which the diameter of aftercooler 216 is about 2inches, clearance 240 is about 0.01 inches. Between housing 226 and agas inlet/outlet tube 246 is a seal 242, for example, an O-ring orferrofluidic warm seal. Pulse tube 222 and regenerator 218 are separatedby vacuum insulation 244.

In use, cold end heat exchanger portion 224 b is directly coupled to arotating machine (not shown) to cool the rotating machine. Flow of highpressure room temperature, helium gas at, for example, 18 atm, betweencompressor 212 and regenerator 218 is controlled by valve assembly 214.The gas pressure is selected to optimize cooler performance. Pulses ofgas are delivered to regenerator 218 and travel through channels 230 and232 to enter pulse tube 222 at a low temperature, for example, about30-80 K. Gas within pulse tube 222 is compressed, followed by expansionwhen valve assembly 214 is actuated to allow reverse flow. The expansionof the gas within pulse tube 222 causes the gas to cool to a lowertemperature, for example, about 20-70 K.

To provide increased system reliability, it is advantageous to haveredundant components in the critical systems, such as the cryogenicrefrigerator, of a high-temperature superconductor device. While thecost of a full redundant refrigeration system including a cold head anda compressor can be cost prohibitive, in a pulse-tube type cryocooler,as the only moving part is the rotary valve assembly which generates thepressure wave, effective redundancy can be obtained by adding a secondvalve assembly connected and controlled such that should a failure occurin the first valve assembly, the second valve assembly takes overcontrol of the system and the operation of the superconducting device isnot disturbed.

The operation of pulse tube refrigerator systems is described forexample in Ishizaki et al, U.S. Pat. No. 5,269,147, and Ohtani et al,U.S. Pat. No. 5,412,952, both incorporated by reference herein in theirentirety. Briefly, in a pulse tube refrigerating systems, a workingfluid contained within a tube is compressed adiabatically by theintroduction of pressurized fluid into the tube causing an increase inthe temperature of the working fluid. Working fluid which has beencompressed passes to a heat exchanger to transfer heat into theatmosphere. The pressurized fluid is then allowed to flow from the tubeand working fluid returns to the tube and expands to decrease intemperature. The cooled working fluid passes to a refrigerating sectionwhere it is available as a coolant. The compression and expansion cycleis repeated.

With reference to FIG. 4, a pulse tube refrigerator system 310 includesa compressor 312, a regenerator 314, and a pulse tube 316. Pulse tube316 includes a cold end heat exchanger 318 and a warm end heat exchanger320. Attached to warm end heat exchanger 320 of pulse tube 316 is abuffer 324.

The flow of high pressure room temperature gas, for example, helium gas,at, for example, 18 atm, between compressor 312 and regenerator 314 iscontrolled by a valve assembly 326, for example, a rotary valveincluding a high pressure flow channel 326 a and a low pressure flowchannel 326 b. Alternatively, valve assembly 326 can include twosolenoid valves. The gas pressure is selected based upon desired systemefficiency. Gas flows from compressor 312 to high pressure flow channel326 a through an inlet line 328, and from low pressure channel 326 b tocompressor 312 through an outlet line 330. High pressure flow channel326 a is controlled to deliver pulses of gas to regenerator 314 througha gas line 332. Gas delivered to regenerator 314 travel through a gasline 334 and enters pulse tube 316 at cold end 318. Gas within a tube336 of pulse tube 316 is compressed, followed by expansion when lowpressure flow channel 326 b is actuated to allow reverse flow throughlines 334 and 332. The expansion of the gas within pulse tube 316 causesthe gas to cool.

Gas flow to and from buffer 324 through a flow line 340 is controlled bya valve 342. Gas flow into and out of warm end heat exchanger 320 ofpulse tube 316 through a flow line 344 is controlled by a valve 346.

The desired reliability in case of system failure is obtained byproviding a back-up valve assembly 356, for example, a rotary valveincluding high and low pressure flow channels 356 a, 356 b,respectively. Alternatively, valve assembly 356 can include two solenoidvalves. Gas flows from compressor 312 to high pressure flow channel 356a through an inlet line 358, and from low pressure flow channel 356 b tocompressor 312 through an outlet line 360. High pressure flow channel356 a is controlled to deliver pulses of gas to regenerator 314 througha gas line 362. Gas within tube 336 expands when low pressure flowchannel 356 b is actuated to allow reverse flow through lines 334 and362.

Opening and closing of flow lines 326 a, 326 b, 356 a and 356 b, as wellas detection of valve failure in valve assembly 326 and switching fromvalve assembly 326 to valve assembly 356, is controlled by controller370.

Located within each of inlet lines 328 and 358 is a solenoid valve 372,374, respectively. Solenoid valve 372 is normally open to allow flowthrough line 328, and solenoid valve 374 is normally closed to preventflow through line 358. Located across each valve assembly 326, 356 is adifferential pressure transducer 376, 378, respectively.

If valve assembly 326 fails, the differential pressure across the valvewill either increase beyond the maximum set value of transducer 376 ordecrease below the minimum set valve of transducer 376. Transducer 376senses the change in pressure and provides a signal to controller 370.In response to the pressure change, controller 370 provides a signal tosolenoid 372 to close and a signal to solenoid 374 to open, therebyswitching from valve assembly 326 to valve assembly 356. Valve assembly356 fuinctions until valve assembly 326 is repaired or changed.

In the compressor system 312, the pump is the most likely component tofail and a second pump can be installed, connected, and controlled toassume operation should the first pump fail, again without disruption tothe superconducting system.

The secondary valve assembly can be used with the pulse tube system ofFIG. 3.

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A cryogenic refrigerator for cooling a rotatingdevice, comprising: a stationary regenerator, and a rotatable cold heatexchanger coupled to the stationary regenerator to rotate relativethereto.
 2. The cryogenic refrigerator of claim 1 wherein the cryogenicrefrigerator is of the Gifford-McMahon type.
 3. The cryogenicrefrigerator of claim 1 further comprising a stationary cylinder housingthe regenerator.
 4. The cryogenic refrigerator of claim 3 furthercomprising a rotatable cylinder mounted to the cold heat exchanger. 5.The cryogenic refrigerator of claim 4 wherein the rotatable cylinder isconcentrically arranged about the stationary cylinder.
 6. The cryogenicrefrigerator of claim 5 further comprising a filler material locatedbetween the stationary and rotatable cylinders.
 7. The cryogenicrefrigerator of claim 4 wherein the rotatable cylinder is axially offsetof the stationary cylinder.
 8. The cryogenic refrigerator of claim 7wherein the cylinders are aligned along a common axis.
 9. The cryogenicrefrigerator of claim 7 further comprising a stem extending from theregenerator.
 10. The cryogenic refrigerator of claim 7 wherein thecylinders define a flow channel therebetween.
 11. The cryogenicrefrigerator of claim 4 further comprising a seal located between thestationary and rotatable cylinders.
 12. The cryogenic refrigerator ofclaim 11 wherein the seal comprises a ferrofluidic seal.
 13. Thecryogenic refrigerator of claim 1 wherein the cryogenic refrigerator isof the pulse-tube type.
 14. The cryogenic refrigerator of claim 1further comprising a pulse tube concentrically arranged relative to theregenerator.
 15. The cryogenic refrigerator of claim 14 wherein thepulse tube is concentrically arranged about the regenerator.
 16. Thecryogenic refrigerator of claim 14 wherein the cold heat exchangerincludes a stationary portion coupled to the regenerator and a rotatableportion coupled to the pulse tube.
 17. The cryogenic refrigerator ofclaim 16 wherein the stationary and rotatable portions of the cold heatexchanger define a flow channel therebetween.
 18. The cryogenicrefrigerator of claim 16 wherein the stationary portion of the cold heatexchanger defines a flow channel.
 19. The cryogenic refrigerator ofclaim 14 wherein the cold heat exchanger includes a screen.
 20. Thecryogenic refrigerator of claim 14 further comprising a surge volumehousing and an aftercooler.
 21. The cryogenic refrigerator of claim 20wherein the surge volume housing and the aftercooler define a floworifice therebetween.
 22. The cryogenic refrigerator of claim 14 furthercomprising a warm end heat exchanger.
 23. The cryogenic refrigerator ofclaim 14 further comprising first and second valve assemblies forcontrolling flow between a compressor and a regenerator of therefrigerator, and a controller for detecting failure in the first valveassembly and switching from the first valve assembly to the second valveassembly.
 24. A cryogenic refrigerator for cooling a rotating device,comprising: a stationary regenerator, a stationary cylinder housing theregenerator, a rotatable cold heat exchanger coupled to the stationaryregenerator to rotate relative thereto, a rotatable cylinder mounted tothe cold end heat exchanger and concentrically arranged about thestationary cylinder, and a ferrofluidic seal located between thestationary and rotatable cylinders.
 25. A cryogenic refrigerator forcooling a rotating device, comprising: a stationary regenerator, astationary cylinder housing the regenerator, a rotatable cold heatexchanger coupled to the stationary regenerator to rotate relativethereto, a rotatable cylinder mounted to the cold end heat exchanger andarranged axially offset of the stationary cylinder along a common axis,the rotatable and stationary cylinders defining a flow channeltherebetween, and a ferrofluidic seal located within the flow channel.26. A cryogenic refrigerator for cooling a rotating device, comprising:a stationary regenerator, a rotatable pulse tube concentrically arrangedabout the regenerator, a cold end heat exchanger including a stationaryportion coupled to the regenerator and a rotatable portion coupled tothe pulse tube, a surge volume housing coupled to the pulse tube torotate therewith, and an aftercooler coupled to the regenerator, thesurge volume housing and the aftercooler defining a flow orificetherebetween.
 27. A method of cooling a rotating superconductor device,comprising: providing a cryogenic refrigerator including a stationaryregenerator and a rotatable cold heat exchanger coupled to thestationary regenerator to rotate relative thereto, and coupling therotatable cold heat exchanger to the superconductor device.